Alanine chloromethylketone and leucine chloromethylketone have been synthesized and shown to inhibit amino acid transport into S37 ascites tumor cells. 14C-leucine chloromethylketone has been found to bind to the plasma membrane fraction of S37 ascites tumor cells. Preliminary purification shows 4 protein fractions specifically labeled. One of these fractions migrates in polyacrylamide gel electrophoresis with gamma-glutamyl transpeptidase activity. Further purification and study of the protein fractions involved in amino acid transport is to be carried out.